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The house mouse Mus musculus is an incredible research tool in the biomedical sciences, due to its ease of care and its ability to be genetically manipulated. Although mice aren't humans, they resemble us closely in many ways, including how insulin signaling works. Genetic manipulation of mice allows researchers to identify biological mechanisms and cause-effect relationships in a very precise manner. One way of doing this is to create "knockout" mice that lack a specific gene, in an attempt to determine that gene's importance in a particular process. Another way is to create transgenic mice that express a gene of interest, often modified in some way. A third method is to use an extraordinary (but now common) tool called "Cre-lox" recombination (1), which allows us to delete or add a single gene in a specific tissue or cell type.

Studying the relationship between obesity and insulin resistance is challenging, because the two typically travel together, confounding efforts to determine which is the cause and which is the effect of the other (or neither). Some have proposed the hypothesis that high levels of circulating insulin promote body fat accumulation*. To truly address this question, we need to consider targeted experiments that increase circulating insulin over long periods of time without altering a number of other factors throughout the body. This is where mice come in. Scientists are able to perform precise genetic interventions in mice that increase circulating insulin over a long period of time. These mice should gain fat mass if the hypothesis is correct.